This proposal requests funds for an investigation of ovarian "fellicular selection." This term refers to the processes which regulate selective growth of only a few follicles in the vertebrate ovary to final maturation and ovulation. The reptilian ovary will be studied because the ovarian follicles are large, arranged in a simple size-gradation, and are susceptible to experimental manipulation. A general hypothesis to explain follicular selection is proposed, i.e., that differential follicular vascularity, augmented by local effects of steroid hormones on ovarian vascularity and gonadotropin binding affiniites, results in biased maturation of some follicles, growth and then atresia of others, and no growth of still others. Various aspects of this hypothesis will be tested. The influence of FSH on follicular growth after partial ovariectomy will give informaion on the capacity of smaller follicles to grow at a supernormal rate. The influence of estradiol and progesterone on FSH-induced growth and maintenance of follicles will be studied. A technique will be developed for implantation of steroids locally into follicles to determine local effects on thecal vascularity and gonadotropin binding affinities. Blood flow rates to follicles of different sizes will be determined using injection of radioactive microspheres. FSH labelled with I131 will be administered to ovaries in vivo and in vitro, and a comparison of FSH binding in the two conditions may indicate if follicular access and sensitivity to gonadotropin is important in follicular selection. Follicular growth patterns and vascularity will also be studied in autografted ovaries. Sympathetic innervation and presence of 3 Beta-hydroxysteroid dehydregenase activity in ovarian follicles of different size will be studied. Finally, follicular size distribution in relation to proximity to stromal vessels will be studied in immature and embryonic ovaries.